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Variola (the virus that causes smallpox) is a member of the orthopoxvirus genus, which also includes viruses that cause cowpox, monkeypox, orf, and molluscum contagiosum. Poxviruses are the largest animal viruses, visible with a light microscope. They are larger than some bacteria and contain double-stranded DNA.
Poxviruses are the only viruses that do not need a cell's nucleus to replicate inside the cell. The variola virus is the only known cause of smallpox. The disease affects only humans. No animal reservoirs or insect vectors (insects that spread a disease) exist, and no carrier state (period when the virus is in the body, but the person is not actively sick) occurs. Before smallpox was wiped out, the disease survived through continual person-to-person transmission. Pregnant women and children had a heightened risk for the illness. Smallpox also affected them more severely than normal. The virus is only transmitted from human to human; there are no known animal infections.
The virus is acquired from inhalation (breathing into the lungs). Virus particles can remain on such items as clothing, bedding, and surfaces for up to one week.
The virus starts in the lungs. From there, the virus invades the bloodstream and spreads to the skin, intestines, lungs, kidneys, and brain. The virus activity in the skin cells creates a rash that starts as macules (flat, red lesions). After this, vesicles (raised blisters) form. Then, pustules (pus-filled pimples) appear about 12-17 days after a person becomes infected. Survivors of smallpox often have severely deformed skin from the pustules.
Christopher J Hogan, MD
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